github: support GCM_ALLOW_UNSAFE_REMOTES option

Author: mjcheetham

src/shared/GitHub/GitHubHostProvider.cs

@@ -285,10 +285,13 @@ public virtual Task EraseCredentialAsync(InputArguments input)
             ThrowIfDisposed();
 
             // We should not allow unencrypted communication and should inform the user
-            if (StringComparer.OrdinalIgnoreCase.Equals(remoteUri.Scheme, "http"))
+            if (!_context.Settings.AllowUnsafeRemotes &&
+                StringComparer.OrdinalIgnoreCase.Equals(remoteUri.Scheme, "http"))
             {
                 throw new Trace2Exception(_context.Trace2,
-                    "Unencrypted HTTP is not supported for GitHub. Ensure the repository remote URL is using HTTPS.");
+                    "Unencrypted HTTP is not recommended for GitHub. " +
+                    "Ensure the repository remote URL is using HTTPS " +
+                    $"or see {Constants.HelpUrls.GcmUnsafeRemotes} about how to allow unsafe remotes.");
             }
 
             string service = GetServiceName(remoteUri);